Many solid golf balls such as two-piece golf balls are known in the art. As compared with the wound golf balls, the solid golf balls have the advantage of an increased total flight distance on both driver and iron shots, because of a so-called straight liner trajectory and a low spin rate due to their structure, which allows for a long run. On the other hand, the solid golf balls are more difficult to control than the wound golf balls in that they do not stop short on the green because of low spin receptivity on iron shots.
Like flight distance, a pleasant feel when hit is essential for golf balls. The absence of a pleasant feel represents a substantial loss of commodity value. As compared with the solid golf balls, the wound golf balls have the structural characteristics ensuring a soft and pleasant feel.
On such solid golf balls, especially two-piece golf balls consisting of a core and a cover, attempts have been made to soften the ball structure in order to accomplish a soft feel upon impact. However, such attempts fail to fully meet the demand. By providing an intermediate layer between the core and the cover, three-piece solid golf balls were proposed. Although many proposals were made, it was still difficult to provide a golf ball having both the flight distance of two-piece solid golf balls and the feel of wound golf balls.
Recently, multi-piece solid golf balls having at least four layers were proposed (see JP-A 9-266959, 10-127818, and 10-127819). One solid golf ball proposed is of the four-layer structure in which a three-layer structure solid core consisting of an internal layer, an intermediate layer and an outer layer is enclosed with a cover. The ball is improved in hitting feel and control by providing a difference in hardness between the respective layers.
However, if the difference in hardness between two adjacent layers is reduced, there is achieved little improvement in hitting feel. If the difference in hardness between two adjacent layers is significant or if the selection of the thickness or material of the adjacent layers is inappropriate, the deflection or deformation upon impact becomes largely different between the adjacent layers. This causes an energy loss at the interface therebetween and thus detracts from resilience, resulting in a reduced flight distance. The durability of the ball against consecutive strikes is also lost. This tendency becomes outstanding particularly when two adjacent layers are formed of different materials which cannot be strongly joined.